Obviously, a good agent is one that makes you money and gets you published and one that you don't find to be the pinnacle of evil. But is there more to it than that? Are there many intangibles about the basis for a good author/agent relationship, or is it all business? I posted the question on AskMetafilter, and here are some of the responses to my question of what authors love about their agents:

As someone who's fired two of them, I would appreciate absolute honesty from them. Don't blow smoke up my butt, tell me that I'm going to be the next big thing, and then not be able to deliver the appropriate (or any) contract. Be honest, be kind, be supportive, be realistic, but most of all, be honest.

I think it's important that they are in touch a lot and always take your calls, or return them right away. Also feeling like they have a stake in your career for the long-term, that they are making decisions based on what is good for you five years from now, not just what will make you a quick buck this month. Crafting your niche or reputation. The big picture. It helps create a feeling of being in a long term business relationship, not just a one-project stand. Also the feeling, (whether or not it's actually true) that they believe in you and your work.

I like her sharp eye, her honest assessment, her great judgment, and of course her excellent connections. (It is really nice to be one of the small fish for an agent who's got big fish other clients.) She's smart and she takes me seriously, and she's looking out for me long-term, not just for the book or proposal at hand. Also, she's someone I'd be friends with even if she weren't my agent. I've been super happy from day one, five years ago.

In doing media work as a lawyer, I developed a view all artist-representing professionals, agents included, that you want to look for the same thing in a professional as in a girlfriend: someone who is almost too good for you. Whatever benefit you might get in priority and enthusiasm from an agent who still hasn't quite made it, is more than offset by that agent's lack of experience and contacts. An agent who is too good for you, though, is as much of a problem. He's often taking you on because he doesn't see where you really are in the market, or as a favor to a more important relationship of his, or for some other reason other than an informed belief that he can, and an avid desire to, earn his 10% off your sales. Something that's almost as important, however, is to recognize that agenting is a very specific skill: selling your work for the best possible price to those who buy work like yours. Hire an agent for those skills, and not for any other attributes (editing, friendship, marketing, career management, etc.)

More thoughts on the topic here.